Revisiting the ARM cut in Compton gamma-ray imaging and its application to the INSPIRE detector

The Compton camera is a gamma-ray imaging device developed in the 1970s. In the 1990s, the COMPTEL detector onboard the CGRO was the first to utilize a Compton camera for MeV all-sky survey observations. Recently, various Compton cameras have been developed using scintillators, semiconductors, and gas detectors, some of which are intended for future small satellite missions as well as medical applications. However, the image obtained by a Compton camera has strong artifacts owing to the overlap of the Compton cones or the arcs, which degrade the resolution and sensitivity of the image. In this study, we revisit the adaptive ARM cut that significantly reduces artifacts when the direction of gamma ray emitting source is already known. This approach complements the statistically well-defined method based on the response function in the three-dimensional data space of scattering direction (χ, ψ) and scattering angle θ, but it is more direct, intuitive, and simplifies the extraction of spectra in astronomical observations of point-like sources. Using a Compton camera, INSPIRE, onboard the ultra-small satellite GRAPHIUM as an example, we numerically evaluated the extent of background reduction to estimate simulation-based sensitivity. The method was also applied to actual measurements using a quarter-scale prototype of INSPIRE to extract spectra from multiple sources within the same field of view.